The present invention relates to a process for the upgrading of hydrocarbons or hydrocarbon streams comprising conjugated diolefins.
Conjugated diolefins such as butadiene, isoprene and the piperylenes are valuable starting materials for various chemical processes, in particular polymerization processes. They are normally obtained by thermal or catalytic cracking processes using naphtha or gas oil as feedstock. The conjugated dienes are normally present in admixture with allenes, alkynes and the corresponding (cyclo) mono-olefins and alkanes, or, when appropriate, as cis-trans mixtures which also may contain allenes, alkynes and the corresponding (cyclo) mono-olefins and alkanes. For polymerization purposes high quality grade conjugated dienes are required. The product mixtures obtained by the cracking process are therefore normally subjected to a further sophisticated treatment (e.g., a distillation-extraction process) so as to warrant good quality products.
It would constitute a considerable advantage when mixtures containing conjugated diolefins, also comprising mono-olefins and alkanes, could be upgraded by a straightforward, low-temperature treatment. It should be noted that the expression "upgrading" as used herein both refers to the partial or total removal of unwanted allenes and alkynes, thus improving the purity of the starting material, as well as to increasing the amount of the more readily polymerizable isomer, thus enlarging the concentration of the desired isomer in the starting material.
Various methods have already been suggested in the art. It is possible, for instance, to subject the hydrocarbon stream comprising conjugated diolefins together with allenic and alkynic impurities (e.g. C.sub.4 -streams ex naphtha cracker) to a hydrogenation treatment to remove such impurities. However, such hydrogenation treatment suffers from a low selectivity in the reduction of the allenes and alkynes since 1,3-diolefins are substantially reduced as well. A distillation treatment would still be required prior to the hydrogenation treatment in order to keep losses in conjugated diolefins within reasonable limits.
The use of sodium to remove impurities has been widely reported in the literature. Much research has been devoted to the upgrading of, in particular, isoprene using sodium and a polymerization inhibitor (e.g. U.S. Pat. Nos. 2,413,254 and 2,398,810), (supported) alkali-amides (e.g. U.S. Pat. Nos. 3,166,605 and 4,060,567) or a sodium dispersion at a discrete higher temperature (German Offenlegungsschrift No. 2,252,846).
It was found, however, that the use of sodium is insufficient with respect to the partial, let alone total removal of allenic impurities without causing unwanted losses of 1,3-dienes, whereas alkynic impurities such as butyne-1 and vinyl-acetylene can be removed easily from 1,3-butadiene using a conventional sodium dispersion.
Surprisingly, it has now been found that allenic impurities can be removed from conjugated dienes by treating such compounds with a dispersion of an alkali(ne earth) metal in the presence of an aliphatic amine having at least one primary amine group. It has further been found that the treatment as described hereinbefore results in an increase in the amount of the trans-isomer of an appropriate conjugated diene which has at least two geometric isomeric forms, starting from all-cis or from a cis/trans mixture.